1. Technical Field
The invention concerns a method of increasing the storage capacity of service information data in a receiver for digital TV transmissions, such as e.g. according to the international MPEG 2 standard and the standards of the European DVB system. The purpose of the increase is to make a larger amount of service information data available for recall, which is used to create an interactive electronic program guide display shown as a so-called xe2x80x9cOn Screen Displayxe2x80x9d (OSD). Such a program guide is designed to make the transmission choice easier and to automate the adjustment of the receiver for the desired transmission. Beyond that the invention can also be used in receivers of other digital transmission systems which transmit other comprehensive information in addition to the actual payload, particularly for a user-friendly guidance.
2. Discussion of Related Art
Digital TV systems simultaneously transmit the video and audio components of several TV programs through a single transmission channel, and additional information if needed from other services by means of a transport time multiplexer. In that case the user can choose from many more programs than were available until now. This makes orientation during the program selection more difficult. Means that were used until now, such as e.g. a program guide in paper form, are not very suitable for clearly presenting the variety of programs and other applications that are offered in parallel, hereafter called services. Help is provided by including more information in the additional data, which are transmitted for control of the system by every transport multiplexer. This additional information is used to display an electronic program guide (EPG) on the screen, which offers the user a dialog for adjusting and programming the receiver.
Each receiver of digital transmissions needs control data for its technical function, such as so-called xe2x80x9cPacket_Identifiersxe2x80x9d (PID) among others, so that when the Packaged Elementary Streams (PES) are demultiplexed from the received transportation stream, the compressed video or audio data contained therein can be functionally assigned. These control data are transmitted as xe2x80x9csectionsxe2x80x9d in the so-called Program Specific Information (PSI), and in the same way as the video and audio elementary streams in the payload part of each transport multiplexer. The system control used in the DVB system conforms to the international standard ISO/IEC 13818-1(1994): xe2x80x9cInformation Technologyxe2x80x94Generic coding of moving pictures and associated audio informationxe2x80x94Part 1: Systemsxe2x80x9d and is therefore assumed to be known. This standard makes it possible to expand the content of the Program Specific Information (PSI) in accordance with the service provider""s needs by means of additional sections, so-called xe2x80x9cPrivate-Sectionsxe2x80x9d.
This expansion is used in the DVB system. In that way the DVB signals also transmit Service Information (SI) in addition to the program specific information (PSI). The service information (SI) contains technical transmission data and particularly describes the Program Elementary Streams (PES) according to origin and content. This makes it possible to automatically tune the receiver for the desired service and provides a user-friendly guide through the variety of the transmitted services by means of the above-mentioned electronic program guide.
The structure of such a system is described in the European telecommunications standard pr ETS 300 468: xe2x80x9cDigital broadcasting systems for television, sound and data services; Specification for Service Information (SI) in Digital Video Broadcasting (DVB) systemsxe2x80x9d, 13.02.1996. In accordance with the latter the PSI data and SI data are transmitted in the same form. Thus a transport multiplexer serially transmits data packets with elementary video streams, elementary audio streams and data streams with PSI or SI data, in a sequence with variable limits. In accordance with the standards both the PSI and the SI data are arranged in the form of tables and subtables, where the tables because of their size can be distributed in several sections of different lengths. The SI tables use the syntax of the PSI standards and contain so-called xe2x80x9cdescriptorsxe2x80x9d to describe the contents and the transmission data. The contents of the SI tables can differ depending on the service provider. A significant component is the included information about services and their contribution to the programs, so-called xe2x80x9ceventsxe2x80x9d of the received distributor network and from other distributor networks. For example by means of so-called SDT xe2x80x9cService Description Tablesxe2x80x9d they contain data for a more detailed description of the individual services, such as among other things the name of the service and the provider, the type of service and its national or regional availability. The data in so-called EIT xe2x80x9cEvent Information Tablesxe2x80x9d is of special significance for a user-friendly program guide. These tables are provided in each transport stream for both the services of the received transport stream and for other transport streams, and in many subtables contain information about the currently running program contributions and subsequent ones, but also the corresponding time schedules for the previewing of contributions during the next hours and days including their starting times, duration, name and description of the contents. Some program providers presently transmit tables for previewing programs during the next seven days.
The magnitude of the necessary amounts of data will be briefly shown by an example. A distributor network with ten transport streams has 80 different services. If all services on the average offer 40 program contributions (events), a weekly preview must make data available for:
80[services]xc2x740eventsxc2x77[days]=22,400events,
thus about 22,400 program contributions. If each program contribution is described with 300 bytes on the average, the description of all program contributions for a week requires event information tables with a total of about 22,400xc2x7300 bytes=6.7 M-bytes. To save the user long waiting time for the desired information, it is desirable for the access time to the entire EIT data to take not more than 10 seconds. To that end it must be periodically and fully transmitted within a period which is clearly under 10 seconds. This would require a data rate of about 5 M-bit/sec just for the transmission of the EIT data. This data rate is not available because of the limited channel width of about 40 M-bit/sec maximum. The event information tables are furthermore provided with different priorities. Thus the tables for the current and subsequent events are transmitted during an essentially shorter time period than those for a complete time overview. For that reason the period for transmitting EIT data for a weekly overview can take up to 30 seconds. Since time overviews that extend beyond four days are generally only transmitted for the current transport stream, the SI data must be obtained from different transport streams. This significantly increases the access time to certain SI sections in the data stream when a weekly overview is used.
A time multiplex transmission like the MPEG 2 method, requires that the sections of the different signal or data components arrive non-continuously at the receiver, thus at different times, and after the multiplexing the data must be stored in the receiver until they are decoded or evaluated. To buffer the different elementary streams, the main control of each receiver has available a transient working memory which provides the corresponding buffer areas. For economical reasons in the past the capacity of the transient working memory was kept relatively small. A capacity of 1 M-byte is generally the standard in practice so that in addition to other tasks, only a relatively small part of about 40 K-bytes is available for service information in a so-called cache area. This area is used e.g. to provide an overview of the running and the subsequent transmissions at all times, independently of the transmission period lengths. Presently however there is already insufficient storage capacity to buffer the above-cited magnitude of EIT data. A user-friendly guidance with a time schedule of at least one week can therefore only be produced if the data that are not stored in the buffer are taken directly from the transport stream. But this can take longer than a minute of waiting time, for the indicated reasons. To correct this deficiency new receivers must be equipped with additional storage capacity, and the customer devices must be refitted with corresponding hardware. This is relatively expensive and therefore unsatisfactory.
The object of the invention is to present a solution which makes it possible to expand the storage capacity, particularly that of the cited cache area, without additional hardware.
The invention makes use of the knowledge that the predominant part of the image memory of the video decoder remains unused during an OSD (on screen display) on the full surface of the receiver screen. During decoding the image memory is predominantly used to predict video images, and is therefore only used to a small extent for the display on the full screen surface. Beyond that the invention utilizes the fact that certain video decoders are connected to the management of the receiver through an interface system, which also makes it possible to read the data that are located in the image memory of the video processor.
To achieve this object, the unoccupied area of the image memory is used by the video decoder as a cache area during the display of graphic pictures on the full screen surface, like those of the electronic program guide, i.e. during a so-called xe2x80x9cFull Screen OSDxe2x80x9d. In order to create an electronic program guide, the microcomputer responsible for the management of the receiver buffers in this cache area the largest possible part of the service information that was extracted from the receiver by a demultiplexer. The microcomputer can instantly recall the parts of the buffered information desired by a user, which avoids the respective waiting time before the next transmission of the corresponding sections.